A process and system for flame detection includes a microprocessor-controlled detector with an array of sensors for sensing radiant energy from a fire or other heat source. A wide band infrared sensor is used as the primary detector, with near band and visible band sensors serving to detect false-alarm energy from non-fire sources. A narrow band sensor, such as a 4.3-micron sensor, may also be employed in the sensor array, and assists in the detection of hydrocarbon fires. Digital signal processing is used to analyze sensed data and discriminate against false alarms. A modulation index of one of the sensors (preferably the wideband IR sensor) is calculated by the microprocessor using measurements of the signal extremes (i.e., maxima and minima). A fire situation is not declared unless the modulation index falls within a prespecified range, such as between 5% and 85%. A multistage alarm system can be provided, which is selectively triggered by the microprocessor. Spectral recording and analysis of prefire data is provided for. The detector can be housed in an enclosed, sealed, removable, plastic housing that may include an integral plastic window lens.